The invention relates generally to a device to switch electrical current, and in particular, to an electron switch that switches electrical current at a high voltage (tens of kilovolts), over sub-nanosecond rise times, and at repetition rates of tens of megahertz.
The cathode ray tube (CRT) was invented by German physicist Karl Ferdinand Braun in 1897. The CRT is the display device that was first used for computer displays, video monitors, televisions, radar displays and oscilloscopes. The CRT developed from Philo Farnsworth's work was used in all television sets until the 1990s and the development of practical plasma screens, liquid crystal display (LCD) televisions, digital light processing (DLP), organic light-emitting diode (OLED) displays, and other technologies. As a result of CRT technology, television has acquired the moniker “the tube” even when referring to non-CRT sets.
A cathode ray tube technically refers to any electronic vacuum tube employing a focused beam of electrons. Cathode rays exist in the form of streams of high speed electrons emitted from the heating of a cathode inside a vacuum tube at its rear end. The emitted electrons form a beam within the tube due to the voltage difference applied across the two electrodes. The beam is then perturbed (deflected) either by a magnetic or an electric field, to trace over (“scan”) the inside surface of the screen (anode). The screen is covered with a phosphorescent coating (often transition metals or rare earth elements), which emits visible light when excited by the electrons.
In television sets and modern computer monitors, the entire front area of the tube is scanned systematically in a fixed pattern called a raster. An image is produced by modulating the intensity of the electron beam with a received video signal (or another signal derived from it). In all modern television sets, the beam is deflected with a magnetic field applied to the neck of the tube with a “magnetic yoke”, a set of wire coils driven by electronic circuits. This usage of electromagnets to change the electron beam's original direction is known as “magnetic deflection”.
The source of the electron beam is the electron gun, which produces a stream of electrons through thermionic emission (also known as the Edison effect), and focuses the electrons into a thin beam. The gun is located in the narrow, cylindrical neck at the extreme rear of a cathode ray tube (CRT), and has electrical connecting pins, usually arranged in a circular configuration, extending from its end. These pins provide external connections to the cathode, to various grid elements in the gun used to focus and modulate the beam, and, in electrostatic deflection CRTs, to the deflection plates. Because the CRT is a hot-cathode device, these pins also provide connections to one or more filament heaters with the electron gun. The electron beam is typically modulated at frequencies of about 1 MHz. The electron beam can also be produced using cold emission. In this case, a single or multiple sharp radius conductors are energized at high enough voltage in vacuum to create an electron emission into vacuum. The electrons are then accelerated similarly to a CRT.
The high voltage (EHT) used for accelerating the electrons is provided by a transformer. For CRTs used in televisions, this is usually a flyback transformer that steps up the line (horizontal) deflection supply to as much as 32 kV for a color tube (Monochrome tubes and specialty CRTs may operate at much lower voltages). The output of the transformer is rectified and the pulsating output voltage is smoothed by a capacitor formed by the tube itself (the accelerating anode being one plate, the glass being the dielectric, and the grounded (earthed) coating on the outside of the tube being the other plate). In the earliest televisions, before the invention of the flyback transformer design, a linear high-voltage supply was used because these supplies were capable of delivering much more current at their high voltage than flyback high voltage systems. However, in the case of an accident, they proved extremely deadly. The flyback circuit design addressed this; in the case of a fault, the flyback system delivers relatively little current, making a person's chance of surviving a direct shock from the high voltage anode lead more hopeful (though by no means guaranteed).
For use in an oscilloscope, the design is somewhat different. Rather than tracing out a raster, the electron beam is directly steered along an arbitrary path, while its intensity is kept constant. In time-domain mode, the usual mode, the horizontal deflection is proportional to time (measured out by a “sweep oscillator” in the oscilloscope, visually progressing across the screen at a constant rate), and the vertical deflection is proportional to the measured signal(s). In the less-common X-Y mode, both the horizontal and vertical deflections are proportional to measured signals. The electron gun is always centered in the tube neck; the problem of ion production is either ignored or mitigated by using an aluminized screen.
Tubes designed for oscilloscope use are longer and narrower than tubes designed for raster scan use, greatly reducing the maximum deflection angle required. This allows for the use of electrostatic deflection instead of magnetic deflection. In this case, deflection is caused by applying an electrical field via deflection plates built into the tube's neck. This method allows the electron beam to be steered much more rapidly than with a magnetic field, where the inductance of the electromagnets imposes relatively severe limits on the maximum frequency in the signal that can be accurately represented. The reduced deflection angle also removes any need for dynamic focusing of the electron beam (which would be difficult to accomplish at the required high deflection speeds). Finally, the limited angle makes it much easier to ensure that the beam deflection produced is a linear function of the signal being traced.
To date, there are no devices that provide a multi-kilovolt pulse of at least 1 kV, a repetition rate exceeding 10 MHz and a nanosecond rise time. Therefore, it is desirable to provide a switching device that produces high-voltage pulses, high-frequency repetition rates and nanosecond rise times.